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Old 02-16-15, 02:36 PM   #13
jeff5may
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Default Just a simple wave of the wand...

Quote:
Originally Posted by Mikesolar View Post
I wish i had the knowledge to write code but I don't.... yet. I am a programming luddite so it is really hard for me.

BBP, could you write one with 1 thermister instead of the pressure sensor and the thermister? If you want to replicate a standard TXV, one would be enough, no?

I looked on the Fujitsu circuit diagrams and do not see a pressure sensor anywhere, just numerous thermisters. As they are just plug in to the board, I don't know if one or two (or three) are used to control the EEV
Here's the catch: the OEM's run their units in test labs for hours and hours with pressure and temperature sensors wired into every part of everywhere and collect volumes of ultra-precise data logs. Even the "basic" capillary tube regulated models are treated this way. These plethora of sensors have no control over the events that transpire in the (torture) testing chamber: they are there as witnesses. The units are put through their paces with eyes on every variable that can be measured.

After a certain amount of testing has transpired, the log data is analyzed. What happens next depends on what type of control system the unit employs.

With a primarily mechanical control system, not a whole lot can be changed beyond tweaking offset adjustments or system component configuration. Due to the static, predictable nature of cap tubes and txv's, they are a "set-it-and-forget-it" type of device.

With the new, variable speed compressor and electronic expansion valve driven systems, this is no longer the case. The control board on these devices has been put through countless hours, months, and years of torture testing, both simulated and real, before a single production unit is released.

The mechanical and refrigeration engineers have had their way with the entire control system many times over. Just because there are only a few thermistors included in the final production model (to save money) does not mean that there aren't secret routines lurking in the abyss that is the control program. When a dog pees on the outdoor unit while it is running, some engineer somewhere has thought about it already and included a countermeasure. When you mismatch the indoor and outdoor units, the brain can tell. Many real-time variables are calculated in the background, based on the model developed in the torture lab. To the average user, the unit just runs like it should until it won't.

A txv uses pressure to regulate superheat. The suction line bulb is charged with very close to the same gas as the refrigeration circuit. The pressure in the bulb follows the saturated suction line pressure as the temperature changes. This pressure acts against one side of the diaphragm that opens or closes the needle valve. The other side of the diaphragm sees either valve discharge pressure (internally equalized) or suction pressure (externally equalized). The adjustment on the valve adds a little closing pressure to the valve to ensure some superheat exists. A subcooling valve works in much the same manner, only the bulb follows saturated condenser pressure and the diaphragm is externally equalized to condenser pressure.

Most of the new EEV units use indoor tube temp, outdoor tube temp, and compressor discharge tube temp thermistors at the least to figure out a partial picture of what's going on with the unit. A great many of them also have indoor and outdoor ambient thermometers also. The inverter units have a startup ritual they go through when the unit is powered up or a mode change occurs. This initialization routine tells the unit how to act. Who knows how much magic is packed in there?
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